Frequent flyers run risk of blood clots, study shows

By Tanya Mohn

Nov. 5, 2007

NEW YORK — Life-threatening blood clots and flying have been linked for more than 50 years, but a new study on business travelers who frequently fly confirms absolute risk for the first time, researchers have said.

People who fly four hours or more have three times the risk of developing clots, compared with periods in which they do not travel, said Frits Rosendaal and Suzanne Cannegieter, of Leiden University Medical Center, The Netherlands, the lead researchers in the study.

A blood clot that forms within large, deep veins of the body, usually in the leg, is called a deep vein thrombosis or DVT. If untreated, part of the clot may break off and travel to the lungs, where it can cause a pulmonary embolism, a potentially fatal condition.

"We now know out of 4,500 people who fly, one will get a DVT within 8 weeks after travel," Cannegieter said. "It's not really a huge amount," but the risk increases with the duration of a flight and the number of flights in a short period. The study tracked almost 9,000 employees of large international companies and organizations during a four- to five-year period. Results were published this fall in the online Journal PLoS Medicine; preliminary findings were reported in late June by the World Health Organization.

Rob Donnelly, vice president of health for Royal Dutch Shell, based in The Hague in the Netherlands, which participated in the study, said the company now used a Web-based tracking system that helped identify at-risk employees, allowing the company to take additional measures if necessary. Employees log on to the company's intranet for training about risks, symptoms and preventive measurers for DVT and other diseases.

The goal is to have awareness without panic, Donnelly said. "This is a manageable risk for the vast majority of people."

Several years ago, Michael Reiff, an American executive with Royal Dutch Shell, landed in the hospital with life-threatening blood clots on his lungs after a flight from Houston to Amsterdam.

Reiff, 53, is of average height and weight, but as head of global compensation and benefits, he took long-haul flights almost 50 percent of the time. The duration and frequency of flights put him at higher risk, according to the study. Flight-induced DVT is often called "economy-class syndrome" because of more cramped seating than in other classes, but it actually occurs in business and first class, the researchers said. Reiff always flew first or business class.

He has since cut his flying time almost in half and no longer sleeps on planes. "Now I literally walk up and down the aisles." At work "I walk the halls every two hours, even at meetings," Reiff said.

Rochelle Broome, corporate medical director of primary care for CHD Meridian Heathcare, which operates 231 on-site medical services for 105 corporate clients, said that in addition to walking, she recommended that employees do frequent seat exercises to ensure increased blood circulation. Many airlines provide examples in flight magazines or in-cabin videos. "You don't have to take a pill. There's no shot. It's easy," said Broome, who experienced several serious DVTs herself.

Keeping well hydrated by drinking a lot of water and limiting alcohol and coffee, and wearing loose-fitting clothing are also common preventive measures.

Broome said that in recent years there had been a heightened awareness of travel-related DVT among client companies with a strong overseas presence. About three years ago, CHD Meridian began a series of initiatives on DVT prevention at both their work site primary care and corporate health centers.

Scott Kalish, a corporate medical director for a CHD client who is also chairman of CHD's travel health committee, said he believed "that many other companies are now implementing programs to make their long distance travelers aware of the risks."

"There is some evidence that the low air pressure in a plane affects the complex coagulation system of the blood," Rosendaal said. When oxygen level goes down, the body may "think" it is losing blood, making it more likely to clot. During sleep, less oxygen is taken in, another reason why sedatives are strongly discouraged.